Step 2: Build It...

1. Using the box cutter, cut out a 6" hole in the bottom of the trash can. Many trash cans have re-rounded circles you can use as guides.

2. Cut out a piece of trash bag a little bit larger than the top of the trash can. Roll up the golf ball in the middle of the bag and secure it with a small bunjee cord. Run a longer bungee cord through the smaller cord.

3. Secure the trash bag to the trash can using tape and attach the longer bunjee cord strap ends to the trash can handles.

Step 3: Use It...

Fill your new Vortex Cannon with smoke, pull back on the bungee/ball handle and let it rip! You should get a nice ring of smoke that shoots across the room. Try a larger hole in the trash can for a larger ring. You've just built the Tub Thumping Cannon! Now on to the Barking Tub Cannon...

Step 4: The Barking Tube Cannon

This Vortex Cannon uses sound waves to generate the smoke rings. This concept was inspired by Bill Beatty.

Step 6: Use It...

You should adjust the Tone Generating software to play a low-frequency saw tooth tone which will 'bump' the woofer and kick enough air to create your computer controlled smoke ring. You can also try different size holes in the tube and different tones and even music for different vortex effects! This is a fun science project and some have been made that really "kick air"! Check out this video. Hope you enjoyed this Instructable.

put a small speaker in bottom of coke can - add flour to coke can - speaker vibration will cause flour to disburse and then cannon will fire it out - if you pot a candle in front of cannon, the cannon will not only blow the candle out - but will ignite the flour in a ring of fire

I believe one factor which is very important is the circularity and finish of the orifice. I'm not sure that you emphasised this sufficiently. If the orifice isn't sufficiently circular, the quality of the vortices and their range will suffer.
You mentioned smoke, but other substances can be used.
For instance rotten-egg gas (hydrogen sulphide from action of an acid on iron sulphide) makes an excellent but exceedingly smelly vortex (that is invisible).
Possibly, one could use a combustible gas and aim the vortex at a source of ignition. PLEASE BE CAREFUL WITH THIS ONE!
Please do not try this one in your home or near flammable materials! If you burn your home down, don't blame me!
I read some of the comments, but in case I missed one that mentioned this, I apologise..